DC-to-DC isolating converters or interleaved converters fed from a rectified AC source typically operate over a large input voltage range and require harmonic input current correction. Effective harmonic correction of a rectified AC supply results in a large amount of ripple current in the secondary circuits of the isolation transformer or such isolating converters. Keeping the output of the converter within acceptable ripple limits has previously required very substantial amounts of storage capacitance. The physical bulk of capacitors satisfying that requirement has heretofore mitigated against the use of effective harmonic correction techniques, particularly in miniaturized and very low output voltage converters.
Known techniques for allowing effective harmonic correction without excessive ripple have included the use of two independent conversion stages, a power factor correcting stage depositing energy on an intermediate high voltage capacitor, or the use of a single stage performing both power factor correction and DC-to-DC conversion. The latter is, however, limited to variable frequency operation or operation only in discontinuous converter states. These operational modes provide efficient operation only for low output power levels.